One approach to sustained delivery of a therapeutic agent is the use of transdermal delivery system such as transdermal patches. Generally, transdermal patches contain a therapeutic agent and an adhesive which allows the transdermal device to adhere to the skin of a patient, allowing for the passage of the active agent from the device through the skin of the patient. Various advantages of using transdermal patches include constant rate of delivery of therapeutic agent, longer duration of action (the ability of the patch to adhere to the skin for 1, 3, 7 days or longer), non-invasive application, improved patient compliance, and the supply of therapeutic agent may be interrupted at any time by tearing off the system. The importance of this means of administration resides in the fact that therapeutic agent can be delivered to the bloodstream without traversing the gastro-intestinal tract and avoiding a “first pass” through the hepatic system prior to reaching the target site. This would avoid any gastrointestinal incompatibility with the pharmaceuticals and unwanted destruction of the pharmaceuticals by metabolism in the gastrointestinal tract. Once the therapeutic agent has penetrated the skin layer, it is absorbed into the blood stream where it can exert a desired pharmacotherapeutic effect. These benefits may be obtained without requiring a professional to administer the therapeutic agent. Transdermal absorption minimizes inter- and intra-patient variations regarding such incompatibilities and metabolisms. By transdermal absorption, it is deemed possible to provide more constant pharmaceutical concentration in the body and to realize a greater pharmaceutical efficiency. It is possible, by proper transdermal absorption, to provide effective dosing of therapeutic agent.
Current pharmaceutical practice provides for transdermal system or transdermal patch that delivers therapeutics at fixed dosages. The rate of delivery of therapeutic agent from the patch to the skin or mucosa of a host, known as the flux rate, is constant and predetermined by the individual patch that is prescribed. The economics of manufacturing and government approval limit the availability of patches with different dosages. Presently, a pharmacist needs to stock multiple patches each containing various dosages of therapeutic agents. For example, a pharmacist needs to stock five different types of transdermal patch, each having dosage strengths such as 25, 50, 75, 100, 150 units per time (micrograms/hour). When a doctor prescribed a certain patch having a certain dosage strength to a patient, the patient purchases enough supply of transdermal patches having the fixed dosage of therapeutic agent. If the prescribed amount is too strong, the patient will have to purchase another supply of transdermal patches having a reduced dosage of therapeutic agent. If the prescribed amount is too weak, the patient will have to purchase another supply of transdermal patches having an increased dosage of therapeutic agent. In this current practice, patches that do not provide the optimum dosage for the patient are being wasted.
Thus, there remains a long felt, yet currently unmet need to provide a transdermal delivery system that can deliver titratable dosage of therapeutic agent. This titratable dosage transdermal delivery system has the advantage of minimizing wastage of transdermal patches that do not provide optimal dosage level of therapeutic agent. There is also a need for a transdermal system which allows fine control of dosage of therapeutic agent to be delivered to a patent. The transdermal delivery system of the present invention solves the problem by providing adjustable dosage of therapeutic agent from a single transdermal delivery system. This invention reduces the number of different types of transdermal patch having various dosage strengths that need to be stocked by a pharmacist. For example, the pharmacist mentioned above would only need to stock two types of transdermal delivery system of the present invention. One type has a dosage of 25 units per patch and the second type has a dosage of 75 units per patch unit. From these two types of patches, it is possible to deliver various dosage strengths such as 25, 50, 75, 100, 150 units. For example, in order to achieve a strength of 50 units, 2 patches of the 25 unit dosage per patch unit may be used. To achieve a strength of 100 units, 4 patches of the same type of patch may be used. To achieve a strength of 150 units, 2 patches of the 75 unit dosage per patch unit may be used. Instead of stocking five different kinds of patches containing different dosages, only two different kinds of patches need to be stocked.